A vented hot water cylinder that loses heat overnight is usually losing it through the pipes at the top, not through the insulation around the store. Hot water rises into the open vent or the draw-off pipe, cools in the loft or cupboard and falls back down the same bore, a one-pipe gravity circulation that drains heat from the top of the cylinder while you sleep. A secondary return loop without a check valve, or with its pump left running, does the same thing around a longer circuit. Some standing loss is normal on any cylinder. This page explains how to tell normal loss from circulation, what the rules in England require of the vent pipe and the insulation, and which checks are safe for you to make.
Why does a vented hot water cylinder lose heat overnight?
A vented hot water cylinder loses heat overnight mainly through the pipes at its top, not through the foam or jacket around it. A vented cylinder is one fed by a cold water cistern in the loft, with an open vent pipe rising from its crown. The hottest water sits at the top of the store, exactly where the vent and the draw-off pipe leave.
Hot water rises into that vertical pipe. It gives up heat to the cooler loft or cupboard and falls back down the same bore. The loop repeats all night, with no pump and no boiler running. Engineers call it gravity circulation, or a one-pipe thermosiphon.
A secondary return can do the same job on a larger scale. That is the small loop that keeps hot water near distant taps. If its pump runs unchecked, or there is nothing stopping reverse flow, the circuit becomes a radiator for the cylinder. A warm airing cupboard at breakfast is the usual clue in both cases.
Is some overnight cooling normal on a vented cylinder?
Some overnight cooling is normal on every vented cylinder. Heatrae Sadia publishes a standing heat loss figure for each capacity of its vented Megalife water heater. The figure rises with the size of the store, so loss is designed in rather than a fault. Standing loss simply means the heat a full, idle cylinder gives off while nobody draws water.
Approved Document L for dwellings in England also limits standing losses for hot water storage vessels. The rate is what matters to you. A cylinder that is merely cooler in the morning is behaving as designed. A cylinder that is cold by morning, with warm pipes above it and a hot cupboard, is losing heat through circulation.
Old insulation changes the picture too. A loose quilted jacket over bare copper loses far more than bonded foam. If the store is at that age, replacement enters the conversation, and converting to an unvented cylinder is one of the routes.
Gravity circulation or the secondary return: which one is cooling your vented cylinder?
A vented hot water cylinder can shed heat overnight through two different movements of water. The table sets gravity circulation up the vent or draw-off against circulation around a secondary return loop.
| What you notice | Gravity circulation up the vent or draw-off | Secondary return circulation |
|---|---|---|
| Which pipe is warm with everything off | The vertical pipe above the cylinder is warm for the first stretch, then cools further up | The small return pipe back to the cylinder stays warm along its whole run |
| Where the heat ends up | In the loft or the cupboard around the cylinder, as hot water rises and cooled water falls in one pipe | Spread around the circuit to distant taps and out of any bare length on the way |
| Is a pump involved | No pump at all: the movement is driven by the temperature difference alone | A bronze circulating pump on the return, which may run when nobody needs hot water |
| What an engineer can change | Pipe routing, a heat trap or a fall in the run, plus insulation at the cylinder connections | A check valve on the return against reverse flow, time control for the pump, and insulation |
| What must never be done | Fitting a valve in the vent pipe: Approved Document G in England requires it open to atmosphere | Leaving the loop bare: Approved Document L asks for all secondary circulation pipework to be insulated |
You can narrow this down without tools. Hours after the boiler and the pump have stopped, check carefully which pipes near the cylinder are still warm. Warm pipework at that point is carrying heat out of the store. Which pipe it is tells the engineer where to start.
Why you cannot fit a valve in the vent pipe to stop the heat loss
The vent pipe of a vented cylinder must stay open to the atmosphere, so a valve is never the cure for overnight heat loss. Approved Document G for England asks for a vent pipe of at least 19 mm internal diameter. It runs from the top of the storage vessel to a point open to the atmosphere above the water level of the cold water cistern.
Heatrae Sadia sets out the same conditions for its vented water heaters. The vent must rise continuously, carry no valves, and may not be replaced by a pressure relief device. Under the Water Supply (Water Fittings) Regulations 1999 in England and Wales, a primary circuit vent must not discharge over a cistern that supplies drinking water.
Stopping gravity circulation is therefore a pipework job, not a shut-off. Re-routing the vent, forming a heat trap and insulating the first run are the legitimate routes, and they are work for a registered engineer. Our guide to hot water cylinder vent pipe regulations covers the detail.
Never fit a valve, cap or restrictor to the vent pipe of a vented cylinder, and never block it. Approved Document G in England requires that pipe to stay open to the atmosphere above the cistern water level. It is the system's escape route if the stored water overheats.
How much insulation Approved Document L expects at a hot water cylinder
Approved Document L for dwellings in England asks for every pipe at a hot water storage vessel to be insulated for at least 1 m from the connection. On a vented cylinder that covers the vent, the draw-off and the cold feed. Those are the three pipes where gravity circulation starts. All secondary circulation pipework should be insulated along its length as well.
The standard is set by heat loss, not by material. Insulated pipes should not exceed the loss figures in BS 5422 for hot water at 60 °C. Table 4.4 of Approved Document L gives minimum thicknesses of high-performance insulation as one way to show that.
- Pipes up to 10 mm: 5 mm of insulation
- Pipes up to 25 mm: 10 mm
- Pipes up to 50 mm: 15 mm
- Pipes up to 100 mm: 20 mm
Replacing a boiler or a cylinder in an existing system pulls the rest of the pipework into scope. Approved Document L asks for all accessible pipes within the dwelling to be insulated when that work is carried out. A new storage vessel should also have electronic temperature control, and hot water circuits should be on a time control separate from the heating.
What you can check yourself when the cylinder goes cold, and what is engineer's work
Four checks are safe for a householder when a vented cylinder keeps going cold overnight. Everything beyond them belongs to a Gas Safe registered engineer.
- Read the boiler pressure gauge and note the figure
- Check the programmer and the thermostats, including the separate hot water time channel
- Press the boiler reset button once if the manufacturer's instructions say to
- Bleed radiators that are cold at the top
The work that actually stops the circulation is not DIY. Changing the run of the vent or draw-off, fitting a check valve on a secondary return, timing the circulating pump, testing the cylinder thermostat and its position, and checking the zone valve and its wiring are all engineer's tasks.
Anything on gas, the flue, seals, condensate inside the appliance or under the boiler casing is work for a Gas Safe registered engineer under the Gas Safety (Installation and Use) Regulations 1998 as amended in England. If the boiler itself has stopped as well, our guide on what to check before you call an engineer runs through the safe checks first.
Overnight cooling and water hygiene: the temperatures HSE expects
HSE guidance for Great Britain expects hot water to be stored above 60 °C and distributed above 50 °C, with cold water below 20 °C. Legionella bacteria multiply between 20 °C and 45 °C. A cylinder that drifts down through the night sits for hours in that band. That is the health reason to stop the circulation rather than simply reheat more often.
HSE also flags dead legs, meaning pipes to outlets that are used rarely or never. The advice is to remove them where possible and to flush little-used outlets at least weekly. Hotter storage raises the risk of scalding at the tap, which HSE addresses with thermostatic mixing valves at the outlet. Our guide on what temperature a hot water cylinder should be set to brings the two together.
If the cylinder and the boiler are both due for renewal, we survey the home free of charge and set out a written fixed price before any work starts for a new boiler installation.
Sources
- Approved Document G: Sanitation, hot water safety and water efficiency (2024 amendments)
- Approved Document L, Conservation of fuel and power, Volume 1: Dwellings (2021, incorporating 2023 amendments)
- The Water Supply (Water Fittings) Regulations 1999, Schedule 2
- HSE – Legionnaires' disease: hot and cold water systems
- HSE – Hot and cold water systems: things to consider
- Heatrae Sadia Megalife vented cistern fed water heater, installation manual